Formula and method
Packs = ceil[area × (1 + waste/100)/coverage per pack]. Total = pack cost + labor on net area + fixed extras.
Worked example
For 30 m² with 10% waste, 2.2 m² packs and a 45 pack price, 15 packs cost 675. Labor at 15/m² and extras of 100 make a total of 1,225.